DIY Swans M3 Tower Speakers Get an Upgrade for 2024!

Posted on July 9, 2024 by Dan

I’ve had a pair of Swans M3 speakers since around the year 2000 – that was 24 years ago as of this writing – they were one of the first pair of really nice speakers I had ever built at that time and were also some of the first speakers to adorn my budding web page called Audio Innovation. I have enjoyed these speakers over the years and they have truly served me well. Whether they were set up for dedicated two-channel listening or as part of a home theater, they have always sounded great. Until a few months ago I started to notice some buzzing from one of the speakers. Upon further inspection I noticed that one of the woofer’s voice coil was rubbing and creating sort of a scratching sound. It was not super obvious, so I didn’t bother with it too much. A while later I noticed that a second woofer was also starting to rub. Okay, so that makes two. Not to mention this also happened in a different pair of speakers I had built around the same time with the W6 woofers. Which I rebuilt with a pair of Dayton Audio RS180s.

I started to consider what it might take to replace the four W6 woofers with something more modern, something newer, something better. As I was researching woofers, I thought I would go ahead and disassemble the speakers and check everything else, make sure there wasn’t something else I needed to replace. I ran a few FR sweeps in REW and that’s when I realized that both of the F5 midrange’s surrounds had broken away and completely separated from the frame! At least 1/3 of the entire surround was completely floating and not connected to anything. Okay, so that has to be fixed. That’s when I turned my focus to the RT1C-A ribbon tweeter. I could see something weird inside the grill on one of them, so I pulled it apart and to my surprise found that the long rectangular magnets on both the frontside and backside had completely disintegrated, turned to basically magnetic dust. Upon disassembly of the other tweeter, the same fate had fallen it, although not quite to the same degree. So if you want to know how long a pair of Swans M3 speakers last, it’s basically less than 24 years. The woofers were toast, the midranges had fallen apart and the tweeters had returned to their natural magnetic form. Dang, this project this got a lot more complicated, but also, a lot more interesting.

So how do you rebuild a pair of Swans M3 speakers in 2024? As it turns out, quite easily actually. HiVi still manufactures and sells both the F5 midrange and the RT1C-A ribbon tweeter. Sadly the W6 woofer is no longer in production. But this is a great start as 1/2 of the drivers are still available, they can be replaced outright without changing anything with the cabinets or the crossover. I had drawn up some different options with other midranges and other tweeters, but felt it changed the classic Swans M3 look that was just so classic of the time back in 2000. If I really wanted to use completely new drivers, I should just go ahead and build a whole new pair of speakers. I wanted to fix these speakers while still maintaining that great look that I’ve grown so accustomed to over the last 24 years. The trick was going to be finding a good woofer that matched the old woofers, but also matched the look and feel of the speakers overall. I tossed around the idea of theDayton Audio RS180s, which I have already in at least 2 other pairs of speakers, so I was sort of tired of building speakers with those drivers again. The newer Signature Series with the flat aluminum dish cones looked like great alternates, but they only offer them in 4 ohm versions. Which would have meant wiring them in series, for an overall less efficient speaker, meaning a completely new crossover and not just for the woofers, but for the mids and tweeters as well. I wasn’t in the mood to completely redesign this thing, so I narrowed my search to only 8-ohm drivers, just like the W6s, so I could wire them in parallel for a 4-ohm load and at least keep most of the crossover in tact (though I would later end up tweaking the crossover anyway, while keeping most of it unchanged).

I wasn’t dead set on maintaining all HiVi drivers initially, until I narrowed my search again to just HiVi drivers and that’s how I landed on the M6N-B 6″ black aluminum/magnesium woofers. These things looked pretty awesome! A quick simulation in Unibox showed they were a perfect fit for a 47 liter boxed tuned to 42 Hz. I wasn’t a huge fan of the stamped steal basket, but otherwise it checked all the boxes. Not to mention they are actually one of the less expensive woofers in the 6-7″ variety. I mocked up a drawing of them paired with the F5 and the RT1C-A and they looked great together, one of the best combinations I had drawn. So with that, I made the decision to buy (4) M6N-B woofers and a new pair of RT1C-A tweeters. I elected not to buy new F5 midranges, but instead bought a small bottle of rubber-infused CA glue and repaired the ones I already had. I bought some rubber feet to clean up the bottom of the cabinets and a few crossover parts so I could tweak the crossover a bit as needed. Since now 24 years later I actually have the ability to measure these speakers and can make changes to the stock crossovers that were originally designed for these speakers.

About this time, I had removed all of the drivers and all of the crossover parts from the cabinet, I decided to refinish the cabinets to make them match some my other speakers I had built recently, like my center channel. This meant sanding down the entire cabinet and refinishing and re-staining them. I ended up going with a darker sort of Walnut look, and repainted the front/top/back in all black. I finished up everything with a few coats of a satin polyacrylic to give the speakers that nice and soft sheen that looks great on any speaker, since it’s not too flashy, but isn’t too dull. It’s sort of the Goldilocks of sheens, it’s just right. With the cabinets all cleaned up, I dropped in the drivers with the crossovers outside of the cabinet and that’s where the fun really began.

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Cerwin Vega L-7 Bookshelf Speaker Crossover-Enclosure-Tweeter Upgrade!

Posted on May 10, 2024 by Dan

Last month I was at my parents house when my dad says, “Hey do you want a pair of speakers?” I thought heck yeah, I’m not one to turn down a free pair of speakers. So he shows me a near perfect-condition pair of vintage Cerwin Vega L-7 bookshelf speakers just sitting on the floor of his bedroom. He said he hasn’t used them in years, they weren’t even connected to anything, and were just sitting around collecting dust. So I absolutely said I would give them a new home and took them off his hands. Almost 30 years ago I had borrowed these same speakers my sophomore year of college and seeing these speakers again instantly brought back memories of blasting Cake and Depeche Mode for all my neighbors to hear. These were decent speakers, even though they don’t have the traditional Cerwin Vega look. Without the badge on the grill, you’d never know who made them. But I always thought they were a good speaker. And now we get a chance to see what this speaker is made of and, of course, see what modifications we can perform to improve their sound and make them even better.

Speaker Breakdown

An external visual of the speaker reveals a 7″ paper cone woofer with rubber surround and a 1/2″ mylar dome tweeter. The cabinet is ported in the rear with a spring-loaded terminal cup. The cabinet is wrapped in a black wood grain veneer, typical of just about any inexpensive speaker available today. The L-7 is fairly lightweight and tapping on the cabinet sides and top reveal a very hollow “knock” indicating little to no internal bracing or damping. Opening up the speaker is quite easy (there are no decorative rings trying to dress up the stamped steal basket of the woofer) which reveals a hollow box with no bracing and basically zero actual acoustic damping, except for a single piece of what looks like a 3/8″ wool fiber pad that had completely dried out and was as stiff as cardboard. At this point we can see that the enclosure is made from 5/8″ particle board and not the more traditional MDF. The crossover is as bare as they come with only 4 components – a single 0.5 mH iron core inductor on the woofer (1st order), a 2.2 uF cap and inductor on the tweeter (2nd order) and a protection device to prevent damage to the tweeter in case of overload. This is extremely bare bones and definitely designed to hit a low price point.

An inspection of the woofer reveals a pretty decent design, although nothing fancy, it appears solid, beefy size magnet, it’s got a cheap stamped steel frame, but the paper cone, spider and surround all look really good. I read somewhere that the drivers were actually made in Germany and not by Cerwin Vega. There is certainly potential here. But the tweeters on the other hand are pretty much garbage. They were very common in cheap speakers back in the day and you can still buy a version of this 1/2″ mylar dome tweeter from Parts Express for about $4. Overall it’s not a great tweeter and will probably be the first thing to go in the upgrade. I measured the speaker with my ECM8000 mic and REW on axis at 0.5m in my room to get an idea of the speaker’s overall tonality and the plots below give you an idea of what we are working with. Cone break-up modes dominate the midrange region right around the crossover point. The sorry tweeter just can’t play low enough to push the crossover point any lower which results in a crossover point of about 5.3 kHz. The tweeter response also can’t keep up with the volume output of the woofer, so it rolls off leaving the midrange to sound even more forward. Overall just not great and clearly designed to be inexpensive and not designed to sound as good as it could. When I was looking up this speaker online I did find that there are two versions of the L-7 with the other version using what’s look like a much better tweeter. Probably a good choice, since besides the hollow particle board box, the tweeter is definitely a major limiting factor to making this a great speaker.

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Converting this $6 Goodwill Vizio Subwoofer into a Bumping Beast!

Posted on February 6, 2024 by Dan

Last weekend I stopped by my local Goodwill hoping to find some audio-related goodies I could play around with. Lately it’s been slim pickins in the used stereo department with mostly junky old Blu-ray players and bits of pieces of all-in-one home theater systems (and those were never any good when they were new). This time I noticed a pretty nice Vizio subwoofer that looked like it had potential. It was at least in good shape. The woofer was hidden behind a grill cover that at the time I couldn’t tell if it was going to be easy to remove. But the nice flared port on the front and the compact size, and decent heft, made me think this just might be a worthwhile buy. It was listed for $12.99, but since it had a blue tag, it just so happened to be a blue tag weekend, which meant it was 50% off. So for a whopping $6.50, I decided it’d be worth taking a shot. If nothing else, the enclosure alone was worth at least that much and I could buy a new driver and amp if I absolutely had to.

Once I got this little subwoofer home I found a model number and looked it up. Turns out it is a Vizio S4251W-B4 Subwoofer that was part of a Vizio S3851W-D4 Soundbar Surround Sound combo back in 2014. I found the old Amazon listing for it and it got surprisingly good reviews. There were no specifications for the sub itself, other that a completely needless output rating of 100 dB. Thanks for that Vizio! So after tearing this little subwoofer down we find a pretty hefty 6″ paper cone subwoofer with a decently fat foam surround. A stamped steal basket and a good-sized magnet make up the motor structure with a label indicating a 4 ohm rating and a 90W power capability. Pressing on the cone revealed it moved just fine with no voice coil rubbing, so at least it didn’t appear to be blown. But man that spider and surround sound were stiff, like this sub has zero compliance. I should have set up REW to measure the T/S parameters (I may still do this later when I measure the amp) but for now I was just checking things out and so far it was looking pretty good.

The enclosure looked decent, made from just 1/2″ MDF, but I really like the long flared port that fired out the front of the cabinet. It reminded me of the old Bose Acoustimass subs from back in the day. Of course this sub just has a regular side-firing driver in a basic 4th-order vented design, but size-wise and visually looked comparable. The amplifier module is mounted in its own smaller enclosure, since they made no attempt to seal off this part of the box, so the subwoofer is in its own enclosure which was really handy for providing options for how we were going to power this thing. So being that it’s part of a soundbar, even though it has an internal amplifier, it doesn’t have any way to connect to it with a regular RCA cable. It’s supposed to connect to the soundbar which most likely uses some kind of proprietary connection. I tried to see if the sub would show up as a Bluetooth device on my phone by using the PAIRING button on the back but to no avail. It wouldn’t show up. Doesn’t really matter, I couldn’t have used it as a Bluetooth device anyway, it just wouldn’t have worked. I needed a direct connection via an RCA to be driven with a regular line-level signal like a normal subwoofer.

I thought there might be a way to bypass the wireless connection portion and get straight to the amp. After poking around this sub actually contains two small chip amps, as the sub was supposed to provide power to a pair of surround sound speakers too. I was able to make out the part number to be a TSA5713 which is a 25W stereo class D chip amp from TI. So let’s take a look at the datasheet and see what we have to work with here. Since there are two amps on this board, one is clearly used in stereo mode (BTL) to drive 25W to each surround sound speaker (assuming they were 4 ohm speakers), which would be connected to the sub via the RCA jacks on the back. The second chip to probably was wired in mono mode (PBTL) to drive the single 4 ohm sub to an unspecified power level. The graph that shows this configuration stops at 40W, just before the amp distortion curve spikes. I’m guessing it can provide around 50W which is respectable, but honestly not great. These little amps are typical of what you find in cheap sound bars and/or TVs and aren’t really anything to write home about. So with that I figured I wouldn’t waste any more time trying to figure out how to tap into a line-level signal and decided just to replace the amp entirely.

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Building a Notch Filter for the Beyerdynamic DT 770 Pro Headphones with Measurements

Posted on January 14, 2024 by Dan

In this article I’ll go over how I designed, built and tested a passive notch filter for the Beyerdynamic DT 770 Pro 250-ohm closed-back headphones and include measurements using REW of the different filters responses. I bought these headphones over Black Friday weekend last year from Beyerdynamic’s website where they were doing a deal where you got a free pair of black Velour EDT 770 VB ear pads, a $40 value, plus the headphones were another $10 off their usual price of $169 and they did free shipping. I was in the market for some new headphones to replace my trusty old Audio-Technica ATH-M30’s that I’ve had for several years and I thought these would do quite nicely. That same weekend my son and I had stopped by Guitar Center and they had about 8 pairs of headphones on display that you could listen to with sample music of varying styles. We checked each of them out and after a quick listen it was pretty obvious the Beyerdynamic DT 770’s were the clear standout. Good bass, nice treble, clean and clear midrange, not to mention they were the most comfortable of the bunch. Before this I hadn’t even considered these headphones, and up until this point I had already picked up a pair of Sennheisers that were also on sale over Black Friday. Though I wasn’t able to A/B these two headphones directly, I decided to return the Sennheisers and picked up the Beyerdynamic DT 770 Pros instead. A decision that would later send me down this rabbit hole of researching filtering and EQ options in order to make these headphones actually listenable for the long term.

While these headphones do sound great overall, they suffer from one major flaw that could be a deal-breaker for most – they are extremely bright! Doing a direct comparison to some of my other headphones, these suckers have got some highs that seriously sparkle, and it’s not subtle. People say that this is just that classic Beyerdynamic sound and I will say that for some tracks, this does sound fantastic and, in short durations, is manageable. But in general, most songs sounds bright and harsh, making long listening expeditions not that enjoyable at all. In all other regards however these headphones sound amazing and they are super comfortable (even while wearing glasses, like I do) so after giving the DT 770s a fair shot straight up, I decided I had to figure out how to EQ these things into submission. Fortunately there’s a ton of info for these headphones on the internet, and I quickly realized I wasn’t alone in my perception of these headphones. And while there are plenty of reviews that will talk about how these headphones sound, I found three major sources to be the most valuable in providing raw measured data that enabled me to tackle this treble problem a little more scientifically.

Naturally my first stop was Rtings.com. Without question, one of the greatest resources for objective reviews of headphones and TVs. They give the DT 770’s an overall Neutral Sound score of 8.3 which is quite high and favorable. Raw frequency response measurements confirm what everyone can hear, that these headphones are extremely neutral (in terms of matching the Harmon target response) from 20 Hz up to almost 6 kHz and therefore have excellent lower-bass response and mid-bass response up through the midrange, but above 6 kHz is where things get a little wonky. The treble climbs well above the target by over 10 dB and peaks at around 8.2 kHz. Even though they provide frequency responses for all their headphones, they don’t offer filter or EQ suggestions on how to improve them. So you’re kind of on your own to figure out how to fix anything that looks amiss. But this is a good start.

That’s when I ran across another incredibly valuable resource for headphones, plus a second set of measurements for the DT 770s, and that was from oratory1990 over on Reddit. oratory1990 has measured response plots for the DT 770s including EQ options to bring them into alignment with the Harmon target. This is where for the first time I downloaded and played around with Equalizer APO and got my first taste of the possibilities for active EQ in Windows. I entered in the EQ settings he recommended and WOW, what a difference! For the first time these headphones sounded absolutely amazing all the way through, that overly bright treble, the sibilance, the shrill, was all gone. I couldn’t believe it was that easy and with that I had figured I had my solution. While I wasn’t a super fan of having to use a software-based EQ, it was powerful enough, and seamless enough, so, why not?

Well, I quickly learned why not, at least for me, that this solution was not a long-term fix. I planned on using these headphones in my mini-budget home recording studio and I quickly realized that Equalizer APO does not work with ASIO drivers and it does not work with live line-in sources, like a piano, microphone or guitar. Basically it doesn’t work with Reaper (my go-to DAW), as it does not EQ anything recorded in it or played through it or anything live via the line in. The EQ only applies only to pre-recorded music played directly in Windows, like with Spotify or YouTube. I thought I could build a similar filter in Reaper using ReaEQ, but quickly learned how much of pain that was as well. Not mention, the biggest gripe about using a software-based EQ was having to turn it off any time I switched to using speakers. So with the Equalizer APO not working for most of my intended applications, I ended up searching for another solution. Now if your setup is more dedicated to just playing back music, or you don’t have speakers, then Equalizer APO is fantastic and I found it to be super useful.

So then last but not least, I came across DIY-Audio-Heaven and as the name suggests, I was in heaven! I have to give mad props to Solderdude who basically inspired this entire project by being the first person (as far as I could tell) to offer a fully passive solution to fixing the Beyerdynamic treble peak. And it’s so simple! I don’t know why I didn’t think of this before. As someone who has designed and built so many freaking speakers, with crossovers, and filters, even notch filters, I don’t know why the idea to passively correct the DT 770s didn’t come to me sooner. But Solderdude absolutely nailed it with his notch filter design, and not to jump ahead too much, this one fix literally solved the treble peak issue with these headphones and has made them sound just incredible. Amazingly enough, he provides passive filter recommendations for hundreds of headphones, not to mention provides measurements and reviews of hundreds of headphones, for your reading pleasure. As it turns out, a lot of headphones that are bright, or sound bad for a myriad of other reasons, can be improved significantly with a single passive in-line filter. And he’s got options for how to fix a lot of them. Not only does he provide this as a service, where you can buy them from him, he basically gives you the design, so if you’re a little more into DIY, then you can tackle this project on your own pretty easily. This was all I needed to start down this path of how to passively EQ the DT 770 Pros to make them sound better.

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Measurement & Review of the AIMIYA A07 Power Amplifier

Posted on December 20, 2023 by Dan

A few months ago I purchased the AIMIYA A07 Power Amplifier from Amazon for a whopping $60 during Prime Day. I’ve had the unit hooked up to my simple studio setup and have been very impressed with it so far. The performance-to-cost ratio is pretty remarkable. It’s worked flawlessly over the past months and I have been very happy with it. I currently have it hooked up to my modded Jamo’s and it sounds absolutely amazing. It has plenty of volume and never seems to run out of power. So today I finally decided to pull out my measurement gear and so I could get some real data on its performance, to see just what this amp is capable of. I know there are plenty of other reviews out there for this amp, part of the reason I purchased it in the first place, so I don’t think I’ll be sharing anything earth shatteringly new. But I enjoyed putting this thing on the bench and getting some data of my own just the same. So read on to see how she performs!

The heart of the AIMIYA A07 is a TI TPA3255 High-power Class D chip amplifier. The datasheet for this part can be found here https://www.ti.com/product/TPA3255. This is a very popular chip amp and has made its way into many small home audio amplifiers from a variety of manufacturers. However, worth noting is implementation of this chip is still up to the designer and AIMIYA is no slouch in this realm touting a long line of amplifier options to suit almost anyone’s need. Overall output power is dictated mostly by how much raw DC power you can provide. AIMIYA supplies a very respectable 32V/5A power supply with the unit which for most people will be plenty, as I’ll show here. But note that that amplifier has more to give, if you’re willing to spend a little more for a larger, more powerful DC power supply. And based on the measurements I took with the stock power supply, I believe more power available in the TPA3255.

So a quick re-cap of my measurement setup (which take it for what it’s worth) is based on a simple loop-back measurement using REW, a PreSonous Firestudio audio interface and a custom balanced amp-level to line-level converter/load box that I designed and built for this exact purpose. I’ve played with this setup quite a bit and have basically got it dialed in as best as I can to take as accurate as possible frequency response and distortion measurements of amps that range from 10W to 400W watts. The Firestudio interface is calibrated (via soundcard cal) and is able to provide an overall THD of only 0.00074% and a THD+N of about 0.0063%. While probably not great for measuring high-performance DACs or headphone amps, this setup is fine for measuring most power amplifiers, or at least most of the amplifiers that I’ve come across, as their distortion levels are almost always much worse that this. I’ve included plots of the baseline loopback measurement of just my setup so you can get an idea of where the noise limitations exist. From this you can see that all of the amplifier measurements are higher than the baseline and are therefore going to be fairly representative of truth for this amp. Not to mention nothing I measured drastically contradicts anything already out there already showing what this amp can do.

So first let’s just look at the frequency response of the A07. One of the easiest things to measure, since REW was basically designed to measure frequency response of just about anything. I limited my measurement to 5-22,000 Hz which shows a very nice and flat response from 20-20,000 Hz where the response is down only -0.3 dB at 20 Hz and down -0.8 dB at 20 kHz. This was measured into a 4 ohm load. I did not do any measurements into an 8 ohm load, as I figured most speakers people are going to use with this amp are going to fall into the 4 ohm territory. Including the Jamo’s that I am current using with it. Basically you can assume the total available power will be lower, and the frequency response will peak slightly before cutoff, but it should be minimal. Overall this frequency response is excellent for a Class D amp and represents a decent filter design by the folks at AIYIMA. Take note that the response does start to roll off at around 5 kHz which when combined with a speaker that if normally sounds a bit bright, this roll-off will mellow it out just a bit, but not much, and will likely go unnoticed. The roll-off in the bass response when combined with the type of speakers that most people will use with this amp (ie, small bookshelf speakers) should also go basically unnoticed. That being said, I’d say this amp is about as neutral as you’d expect and should sound great with a majority of speakers.

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Inexpensive Crossover Upgrade for the OSD Nero MB5 Bookshelf Speakers

Posted on August 6, 2023 by Dan

So I picked up a pair of OSD Nero MB5 bookshelf speakers about a month ago and after giving them a listen and measuring their frequency response, I decided these speakers need an upgrade! In a previous post I updated a pair of Jamo Concert 9 Series speakers where we tackled internal damping, port tuning and the crossover. In this update we’re just going to be tackling the crossover with just one very minor change to the damping. First off I think the Nero MB5’s are great little speakers. I got them on sale over at Crutchfield for only $99. Amazon typically has them for around $159. As of this writing they are still on sale at Crutchfield and for this price, in combination with the updates we’re about to go over, these speakers honestly cannot be beat. These are essentially the passive version of the Nero AB5 powered speakers, ditching the internal amplification (and any DSP) for a simple passive solution with a passive crossover. They even left the LED for the logo on the front baffle, but it isn’t connected to anything.

They have the look of a nice studio monitor, but without the finesse or feel. The box is light, lacks any real bracing and any significant damping, save a single piece of poly fill that covers about half of each side and the top. The soft dome tweeter is framed in a very good-looking waveguide which provides a classic 1st order gain response as the frequency decreases, but any benefit here is not really realized due to the paltry 6 dB/octave filter on the woofer which lets an excessive amount of cone break-up modes to enter the equation. The dual ports provide a tuning frequency of about 60 Hz which is fantastic for a speaker this size. The little 5-1/4″ woofer can produce an impressive amount of bass and depending on how close you set these against a wall, the bass can either be just right or way too much. Don’t expect miracles here, but the bass response is quite punchy. Some port noise (chuffing) can be heard for bass-heavy tracks that don’t have a lot of other music to mask it. But is bearable.

At first listen I really enjoyed these speakers, there was nothing that appeared completely wrong or unmanageable. After a quick measurement with REW and my Behringer ECM8000 mic, we start to see some of the issues this speaker actually has. Measured on axis at about 0.5m directly in front of the speaker midway between the woofer and tweeter, we see a 6-7 dB peak around 5 kHz that screws up any hope of expecting these speakers to sound flat or neutral. Note that this response will look different depending on listening position. Move up or down or left or right and this peak is less predominant. However for purposes of this crossover upgrade, the intent was to make the speaker flat on-axis and ear-level being directly between the woofer and tweeter. As it stands, these speakers will sound a little better either above or below the midpoint of this speaker. But for now let’s open this speaker up and see what makes it tick and see if we can figure out what is causing this uneven frequency response and what it might take to make it better.

Disassembly of the Nero MB5 is pretty straightforward. There are a total of (12) 4mm hex wood screws holding the plastic front baffle and terminal cup/crossover. The only real trick is there are two smaller phillips-head screws hidden underneath the 3M EVA vibe isolation pad in the front left and right corners. You have peel back the foam over about a 1″ x 1″ area to access the screw to remove it, thus releasing a plastic tab attached to the front baffle. This seems holey unnecessary and just makes removal of the front baffle trickier. See pics below for an idea of what this looks like. Once that’s out of the way, the front baffle and as it turns out, the tweeter assembly, come off together. The 5-1/4″ woofer is revealed and can be removed as well. After disconnecting the drivers we can remove the terminal cup and crossover assembly and get our first look at what kind of magic the folks at OSD came up with to tame this beast. Turns out to be just another typical cheap crossover consistent with this type of speaker at this price point. The low-pass filter duties are handled by a basic iron-core inductor creating a simple 1st order, 6dB/octave filter network. Boring, and, ineffective.

I have, in my experience, never found a 1st order filter to be sufficient for a low-pass design to suppress cone break-up modes to a manageable level. The best you might get is with a poly cone as they tend to have slightly better break-up modes, but OSD touts this is a “composite fiberglass woofer” and you aren’t breaking any laws of physics here. Once you get above the fundamental 1st mode of this material, it’s going to resonate, its going to break up, it’s going to like being excited at these frequencies and will respond in kind. If you’re gonna design a speaker and expect to get away with a 1st order filter (which by the way can be turned into a 2nd order filter with a cheap-o 79-cent capacitor), you better be prepared to work out some exotic cone material with specialized composites for better damping to control those break-up modes or you’re going to be fighting a losing battle. 1st order filters just don’t have what it takes to be effective in this regard. So that’s the first thing we will fix. The woofer measured about 3.6 ohms at DC and therefore represents a solid 4 ohm speaker. See the impedance plot of the woofer measured in the box below.

Moving on to the tweeter, we have a 2nd order high-pass filter made up of a 1.2 uF poly cap, a 0.24 mH inductor and an 8 ohm series resistor. This actually sets the crossover frequency to be fairly high, you’d expect a gap going from the woofer to the tweeter, however that’s where the waveguide comes in and does its magic. The waveguide is providing about 6 dB/octave gain all the way down to about 2 kHz. When combined with the filter response, you end up with a decent overall frequency response that actually combines really well with the woofer. And the benefit is that this gain comes acoustically, so the tweeter is able to do less, i.e., less distortion, and still provide a decent output. The only adjustments needed here will be to match the crossover point and level with the woofer once we add the 2nd order filter to the woofer. We’re going to make only a small adjustment here, as I felt like the raw tweeter response with the waveguide was quite good with no serious defects. In which case all we need to do is set the crossover point so it sums well with the woofer and adjust its level so it’s not too bright but not too dull. Special notch or other shaping filters won’t be needed here.

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